Apparatus for withdrawing permeate using an immersed vertical skein of hollow fibre membranes

ABSTRACT

An apparatus for treating a multi component liquid substrate while leaving particulate matter therein as a skein of hollow fiber filtering membranes immersed in the substrate which is contained in a non-pressurized reservoir. A pumping fluid communication with the lumens of the membranes draws a component of the substrate as permeate through the membranes by applying a section to the lumens of the membranes. In various embodiments, an aeration system as a gas distributor for discharging air directly into the substrate within the skein, upper and lower headers of the skein are spaced apart by a gas tube, and a gas distribution system has through passages through the lower header to discharge bubbles into the substrate above the lower header.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 09/507,438, filed Feb.19, 2000 issued as U.S. Pat. No. 6,294,039; which is a division of Ser.No. 09/258,999, filed Feb. 26, 1999, issued as U.S. Pat. No. 6,042,677;which is a division of Ser. No. 08/896,517, filed Jun. 16, 1997, issuedas U.S. Pat. No. 5,910,250; which is a continuation-in-part applicationof Ser. No. 08/690,045, filed Jul. 31, 1996, issued as U.S. Pat. No.5,783,083 which is a non-provisional of provisional application Ser. No.60/012,921 filed Mar. 5, 1996 and a continuation-in-part of Ser. No.08/514,119, filed Aug. 11, 1995, issued as U.S. Pat. No. 5,639,373. Thedisclosure of all the patents and applications listed above are herebyincorporated by reference thereto as if fully set forth herein.

FIELD OF THE INVENTION

Background of the Invention

This invention relates to a membrane device which is an improvement on aframeless array of hollow fiber membranes and a method of maintainingclean fiber surfaces while filtering a substrate to withdraw a permeate,which is also the subject of U.S. Pat. No. 5,248,424; and, to a methodof forming a header for a skein of fibers. The term “vertical skein” inthe title (hereafter “skein” for brevity), specifically refers to anintegrated combination of structural elements including (i) amultiplicity of vertical fibers of substantially equal length; (ii) apair of headers in each of which are potted the opposed terminalportions of the fibers so as to leave their ends open; and, (iii)permeate collection means held peripherally in fluid-tight engagementwith each header so as to collect permeate from the ends of the fibers.

The term “fibers” is used for brevity, to refer to “hollow fibermembranes” of porous or semipermeable material in the form of acapillary tube or hollow fiber. The term “substrate” refers to amulticomponent liquid feed. A “multi-component liquid feed” in this artrefers, for example, to fruit juices to be clarified or concentrated;wastewater or water containing particulate matter; proteinaceous liquiddairy products such as cheese whey, and the like. The term “particulatematter” is used to refer to micron-sized (from 1 to about 44 μm) andsub-micron sized (from about 0.1 μm to 1 μm) filtrable matter whichincludes not only particulate inorganic matter, but also dead and livebiologically active microorganisms, colloidal dispersions, solutions oflarge organic molecules such as fulvic acid and humic acid, and oilemulsions.

The term “header” is used to specify a solid body in which one of theterminal end portions of each one of a multiplicity of fibers in theskein, is sealingly secured to preclude substrate from contaminating thepermeate in the lumens of the fibers. Typically, a header is acontinuous, generally rectangular parallelpiped of solid resin(thermoplastic or thermosetting) of arbitrary dimensions formed from anatural or synthetic resinous material. In the novel method describedhereinbelow, the end portions of individual fibers are potted inspaced-apart relationship in cured resin, most preferably by “potting”the end portions sequentially in at least two steps, using first andsecond potting materials. The second potting material (referred to as“fixing material”) is solidified or cured after it is deposited upon a“fugitive header” (so termed because it is removable) formed bysolidifying the first liquid. Upon removing the fugitive header, what isleft is the “finished” or “final” header formed by the second pottingmaterial. Of course, less preferably, any prior art method may be usedfor forming finished headers in which opposed terminal end portions offibers in a stack of arrays are secured in proximately spaced-apartrelationship with each other.

The '424 patent required potting the opposed ends of a frameless arrayof fibers and dispensed with the shell of a module; it was animprovement on two preceding configurations disclosed in U.S. Pat. Nos.5,182,019, and 5,104,535, each of which used frameless arrays andavoided potting the fibers. The efficiency of gas-scrubbing a '424 arraywas believed to be due, at least in large part, to a substantial portionof the fibers of the fibers in the array lying in transverserelationship to a mass of rising bubbles, referred to herein as a“column of rising bubbles”, so as to intercept the bubbles. Specificexamples are illustrated in FIGS. 9, 9A, 10 and 11 of the '424 patent.

A '424 “array” referred to a bundle of arcuate fibers the geometry ofwhich array was defined by the position of a pair of transversely spacedheaders in which the fibers were potted. In the '424 array, as in thearray of this invention, each fiber is free to move independently of theothers, but the degree of movement in the '424 is unspecified andarbitrary, while in the vertical skein of this invention, movement iscritically restricted by the defined length of the fibers betweenopposed headers. Except for their opposed ends being potted, there is nophysical restraint on the fibers of a skein. To avoid confusion with theterm “array” as used for the '424 bundle of arcuate fibers, the term“skein fibers” is used herein to refer to plural arrays. An “array” inthis invention refers to plural, essentially vertical fibers ofsubstantially equal lengths, the one ends of each of which fibers areclosely spaced-apart, either linearly in the transverse (y-axis herein)direction to provide at least one row, and typically plural rows ofequidistantly spaced apart fibers. Less preferably, a multiplicity offibers may be spaced in a random pattern. Typically, plural arrays arepotted in a header and enter its face in a generally x-y plane (see FIG.5). The width of a rectangular parallelpiped header is measured alongthe x-axis, and is the relatively shorter dimension of the rectangularupper surface of the header; and, the header's length, which is itsrelatively longer dimension, is measured along the y-axis.

This invention is particularly directed to relatively large systems forthe microfiltration of liquids, and capitalizes on the simplicity andeffectiveness of a configuration which dispenses with forming a modulein which the fibers are confined. As in the '424 patent, the novelconfiguration efficiently uses a cleansing gas, typically air,discharged near the base of a skein to produce bubbles in a specifiedsize range, and in an amount large enough to scrub the fibers, and tocause the fibers to scrub themselves against one another. Unlike in the'424 system the fibers in a skein are vertical and do not present anarcuate configuration above a horizontal plane through the horizontalcenter-line of a header. As a result, the path of the rising bubbles isgenerally parallel to the fibers and is not crossed by the fibers of avertical skein. Yet the bubbles scrub the fibers. The restrictedlyswayable fibers, because of their defined length, do not get entangled,and do not abrade each other excessively, as is likely in the '424array. The defined length of the fibers herein minimizes (i) shearingforces where the upper fibers are held in the upper header, (ii)excessive rotation of the upper portion of the fibers, as well as (iii)excessive abrasion between fibers. The fibers of this invention areconfined so as to sway in a “zone of confinement” (or “bubble zone”)through which bubbles rise along the outer surfaces of the fibers. Theside-to-side displacement of an intermediate portion of each fiberwithin the bubble zone is restricted by the fiber's length. The bubblezone, in turn, is determined by one or more columns of vertically risinggas bubbles, preferably of air, generated near the base of a skein.

Since there is no module in the conventional sense, the main physicalconsiderations which affect the operation of a vertical skein in areservoir of substrate relate to intrinsic considerations, namely, (a)the fiber chosen, (b) the amount of air used, and (c) the substrate tobe filtered. Such considerations include the permeability and rejectionproperties of the fiber, the process flow conditions of substrate suchas pressure, rate of flow across the fibers, temperature, etc., thephysical and chemical properties of the substrate and its components,the relative directions of flow of the substrate (if it is flowing) andpermeate, the thoroughness of contact of the substrate with the outersurfaces of the fibers, and still other parameters, each of which has adirect effect on the efficiency of the skein. The goal is to filter aslow moving or captive substrate in a large container under ambient orelevated pressure, but preferably under essentially ambient pressure,and to maximize the efficiency of a skein which does so (filters)practically and economically.

In the skein of this invention, all fibers in the plural rows of fibers,staggered or not, rise generally vertically while fixedly held neartheir opposed terminal portions in a pair of opposed, substantiallyidentical headers to form the skein of substantially parallel, verticalfibers. This skein typically includes a multiplicity of fibers, theopposed ends of which are potted in closely-spaced-apart profusion andbound by potting resin, assuring a fluid-tight circumferential sealaround each fiber in the header and presenting a peripheral boundaryaround the outermost peripheries of the outermost fibers. The positionof one fiber relative to another in a skein is not critical, so long asall fibers are substantially codirectional through one face of eachheader, open ends of the fibers emerge from the opposed other face ofeach header, and substantially no terminal end portions of fibers are infiber-to-fiber contact. We found that the skein of fibers, deployed tobe restrictedly swayable, were as ruggedly durable as they were reliablein operation.

The fibers are stated to be “restrictedly swayable”, because the extentto which they may sway is determined by the free length of the fibersrelative to the fixedly spaced-apart headers, and the turbulence of thesubstrate. When a large number of fibers is used in a skein, as istypically the case herein, the movement of a fiber adjacent to othersmay be modulated by the movement of the others, but the movement offibers within a skein is constricted. This system is therefore limitedto the use of a skein of fibers having a critically defined lengthrelative to the vertical distance between headers of the skein. Thedefined length limits the side-to-side movement of the fibers in thesubstrate in which they are deployed, except near the headers wherethere is negligible movement.

In the prior art, a vertical skein of fibers in a substrate is typicallyavoided due to expected problems relating to channelling of the feed.However, because the fibers are restrictedly swayable in a “bubble zone”as described herebelow, the fibers are substantially evenly contactedover their individual surfaces with substrate and provide filtrationperformance based on a maximized surface which is substantially the sumof the surface areas of all fibers in contact with the substrate.Moreover, because of the ease with which the substrate coats thesurfaces of the vertical fibers in a skein, and the accessibility ofthose surfaces by air bubbles, the fibers may be densely arranged in aheader to provide a large membrane surface of up to 1000 m² and more.

One header of a skein is displaceable in any direction relative to theother, either longitudinally (x-axis) or transversely (y-axis), onlyprior to the headers being vertically fixed in spaced apart parallelrelationship within a reservoir, for example, by mounting one headerabove another, against a vertical wall of the reservoir which functionsas a spacer means. This is also true prior to spacing one header aboveanother with other spacer means such as bars, rods, struts, I-beams,channels, and the like, to assemble plural skeins into a “bank ofskeins”(“bank” for brevity), in which bank a row of lower headers isdirectly beneath a row of upper headers. After assembly into a bank, asegment intermediate the potted ends of each individual fiber isdisplaceable along either the x- or the y-axis, because the fibers areloosely held in the skein. There is essentially no tension on each fiberbecause the opposed faces of the headers are spaced apart at a distanceless than the length of an individual fiber.

By operating at ambient pressure, mounting the headers of the skeinwithin a reservoir of substrate, and by allowing the fibers restrictedmovement within the bubble zone in a substrate, we minimize damage tothe fibers. Because, a header secures at least 10, preferably from 50 to50,000 fibers, each generally at least 0.5 m long, in a skein, itprovides a high surface area for filtration of the substrate.

The fibers divide a reservoir into a “feed zone” and a withdrawal zonereferred to as a “permeate zone”. The feed of substrate is introducedexternally (referred to as “outside-in” flow) of the fibers, andresolved into “permeate” and “concentrate” streams. The skein, or a bankof skeins of this invention is most preferably used for microfiltrationwith “outside-in” flow. Typically a bank is used in a relatively largereservoir having a volume in excess of 10 L (liters), preferably inexcess of 1000 L such as a flowing stream, more typically a reservoir(pond or tank). Most typically, a bank or plural banks with collectionmeans for the permeate, are mounted in a tank under atmosphericpressure, and permeate is withdrawn from the tank.

Where a bank or plural banks of skeins are placed within a tank orbioreactor, and no liquid other than the permeate is removed the tank isreferred to as a “dead end tank”. Alternatively, a bank or plural banksmay be placed within a bioreactor, permeate removed, and sludge disposedof; or, in a tank or clarifier used in conjunction with a bioreactor,permeate removed, and sludge disposed of.

Operation of the system relies upon positioning at least one skein,preferably a bank, close to a source of sufficient air or gas tomaintain a desirable flux, and, to enable permeate to be collected fromat least one header. A desirable flux is obtained, and provides theappropriate transmembrane pressure differential of the fibers underoperating process conditions. “Transmembrane pressure differential”refers to the pressure difference across a membrane wall, resulting fromthe process conditions under which the membrane is operating.

The relationship of flux to permeability and transmembrane pressuredifferential is set forth by the equation:

J=k▴P

wherein, J=flux; k=permeability constant; ▴P=transmembrane pressuredifferential; and k=1/μRm where μ=viscosity of water and, Rm=membraneresistance.

The transmembrane pressure differential is preferably generated with aconventional non-vacuum pump if the transmembrane pressure differentialis sufficiently low in the range from 0.7 kPa (0.1 psi) to 101 kPa (1bar), provided the pump generates the requisite suction. The term“non-vacuum pump” refers to a pump which generates a net suction sidepressure difference, or, net positive suction head (NPSH), adequate toprovide the transmembrane pressure differential generated under theoperating conditions. By “vacuum pump” we refer to one capable ofgenerating a suction of at least 75 cm of Hg. A pump which generatesminimal suction may be used if an adequate “liquid head” is providedbetween the surface of the substrate and the point at which permeate iswithdrawn; or, by using a pump, not a vacuum pump. A non-vacuum pump maybe a centrifugal, rotary, crossflow, flow-through, or other type.Moreover, as explained in greater detail below, once the permeate flowis induced by a pump, the pump may not be necessary, the permeatecontinuing to flow under a “siphoning effect”. clearly, operating withfibers subjected to a transmembrane pressure differential in the rangeup to 101 kPa (14.7 psi), a non-vacuum pump will provide adequateservice in a reservoir which is not pressurized; and, in the range from101 kPa to about 345 kPa (50 psi), by superatmospheric pressuregenerated by a high liquid head, or, by a pressurized reservoir.

The fibers are not required to be subjected to a narrowly criticaltransmembrane pressure differential though fibers which operate under asmall transmembrane pressure differential are preferred. A fiber whichoperates under a small transmembrane pressure differential in the rangefrom about 0.7 kPa (0.1 psi) to about 70 kPa (10 psi) may producepermeate under gravity alone, if appropriately positioned relative tothe location where the permeate is withdrawn. In the range from 35 kPa(0.5 psi) to about 206 kPa (30 psi) a relatively high liquid head may beprovided with a pressurized vessel. The longer the fiber, which greaterthe area and the more the permeate.

In the specific instance where a bank is used in combination with asource of cleansing gas such as air, both to scrub the fibers and tooxygenate a mixed liquor substrate, most, if not all of the airrequired, is introduced either continuously or intermittently, near thebase of the fibers near the lower header. The perforations through whichthe gas is discharged near the header are located close enough to thefibers so as to provide columns of relatively large bubbles, preferablylarger than about 1 mm in nominal diameter, which codirectionallycontact the fibers and flow vertically along their outer surfaces,scrubbing them. The outer periphery of the columns of bubbles define thezone of confinement in which the scrubbing force exerted by the bubbleson the fibers, keeps their surfaces sufficiently free of attachedmicroorganisms and deposits of inanimate particles to provide arelatively high and stable flow of permeate over many weeks, if notmonths of operation. The significance of this improvement will be betterappreciated when it is realized that the surfaces of fibers inconventional modules are cleaned nearly every day, and sometimes moreoften.

Because this system, like the '424 system, does away with using a shell,there is no void space within a shell to be packed with fibers; and,because of gas being introduced proximately to, and near the base ofskein fibers, there is no need to maintain a high substrate velocityacross the surface of the fibers to keep the surfaces of the fibersclean. As a result, there is virtually no limit to the number ofrestrictedly swayable fibers which may be used in a skein, the practicallimit being set by (i) the ability to pot the ends of the fibersreliably; (ii) the ability to provide sufficient air to the surfaces ofessentially all the fibers, and (iii) the number of banks which may bedeployed in a tank, pond or lake, the number to be determined by thesize of the body of water, the rate at which permeate is to bewithdrawn, and, the cost of doing so.

Typically, a relatively large number of long fibers, at least 100, isused in a skein of restrictedly swayable fibers, the fibers operateunder a relatively low transmembrane pressure differential, and permeateis withdrawn with a non-vacuum pump. If the liquid head, measured as thevertical distance between the level of substrate and the level fromwhich permeate is to be withdrawn, is greater than the transmembranepressure differential under which the fiber operates, the permeate willbe separated from the remaining substrate, due to gravity.

Irrespective of whether a non-vacuum pump, vacuum pump, or other type ofpump is used, or permeate is withdrawn with a siphoning effect, it isessential that the fibers in a skein be positioned in a generallyvertical attitude, rising above the lower header. An understanding ofhow a vertical skein operates will make it apparent that, since fibersin a skein are anchored at the base of the skein by the lower header,the specific gravity of the fibers relative to that of the substrate isimmaterial and will not affect their vertical disposition.

The unique method of forming a header disclosed herein allows one toposition a large number of fibers, in closely-spaced apart relationship,randomly relative to one another, or, in a chosen geometric pattern,within each header of synthetic resinous material. It is preferred toposition the fibers in arrays before they are potted to ensure that thefibers are spaced apart from each other precisely, and, to avoid wastingspace on the face of a header; it is essential, for greatestreliability, that the fibers not be contiguous. By sequentially pottingthe terminal portions of fibers in stages as described herein, thefibers may be cut to length in an array, either after, or prior to beingpotted. The use of a razor-sharp knife, or scissors, or other cuttingmeans to do so, does not decrease the open cross-sectional area of thefibers' bores (“lumens”). The solid resin forms a circumferential sealaround the exterior terminal portions of each of the fibers, open endsof which protrude through the permeate-discharging face of each header,referred to as the “aft” face.

Further, one does not have to cope with the geometry of a frame, thespecific function of which is to hold fibers in a particular arrangementwithin the frame. In a skein, the sole function of the header spacingmeans is to maintain a fixed vertical distance between headers which arenot otherwise spaced apart. In a skein of this invention, there is noframe.

The skein of this invention is most preferably used to treat wastewaterin combination with a source of an oxygen-containing gas which isbubbled within the substrate, near the base of a lower header, eitherwithin a skein or between adjacent skeins in a bank, for the specificpurpose of scrubbing the fibers and oxygenating the mixed liquor inactivated sludge, such as is generated in the bioremediation ofwastewater. It was found that, as long as enough air is introduced nearthe base of each lower header to keep the fibers awash in bubbles, andthe fibers are restrictedly swayable in the activated sludge, a build-upof growth of microbes on the surfaces of the fibers is inhibited whilepermeate is directly withdrawn from activated sludge, and excellent flowof permeate is maintained over a long period. Because essentially allsurface portions of the fibers are contacted by successive bubbles asthey rise, whether the air is supplied continuously or intermittently,the fibers are said to be “awash in bubbles.”

The use of an array of fibers in the direct treatment of activatedsludge in a bioreactor, is described in an article titled “DirectSolid-Liquid Separation Using Hollow Fiber Membrane in an ActivatedSludge Aeration Tank” by Kazuo Yamamoto et al in Wat. Sci. Tech. Vol.21, Brighton pp 43-54, 1989, and discussed in the '424 patent, thedisclosure of which is incorporated by reference thereto as if fully setforth herein. The relatively poor performance obtained by Yamamoto et alwas mainly due to the fact that they did not realize the criticalimportance of maintaining flux by aerating a skein of fibers from withinand beneath the skein. They did not realize the necessity of thoroughlyscrubbing substantially the entire surfaces of the fibers by flowingbubbles through the skein to keep the fibers awash in bubbles. Thisrequirement becomes more pronounced as the number of fibers in the skeinincreases.

As will presently be evident, since most substrates are contaminatedwith micron and submicron size particulate material, both organic andinorganic, the surfaces of the fibers in any practical membrane devicemust be maintained in a clean condition to obtain a desirable specificflux. To do this, the most preferred use of the skein as a membranedevice is in a bank, in combination with a gas-distribution means, whichis typically used to distribute air, or oxygen-enriched air between thefibers, from within the skein, or between adjacent skeins, at the basesthereof.

Tests using the device of Yamamoto et al indicate that when the air isprovided outside the skein the flux decreases much faster over a periodof as little as 50 hr, confirming the results obtained by them. This isevident in FIG. 1 described in greater detail below, in which the graphsshow results obtained by Yamamoto et al, and the '424 array, as well asthose with the vertical skein, all three assemblies using essentiallyidentical fibers, under essentially identical conditions.

The investigation of Yamamoto et al with downwardly suspended fibers wascontinued and recent developments were reported in an article titled“Organic Stabilization and Nitrogen Removal in Membrane SeparationBioreactor for Domestic Wastewater Treatment” by C. Chiemchaisri et aldelivered in a talk to the Conference on Membrane Technology inWastewater Management, in Cape Town, South Africa, Mar. 2-5, 1992, alsodiscussed in the '424 patent. The fibers were suspended downwardly andhighly turbulent flow of water in alternate directions, was essential.

It is evident that the disclosure in either the Yamamoto et al or theChiemchaisri et al reference indicated that the flow of air across thesurfaces of the suspended fibers did little or nothing to inhibit theattachment of microorganisms from the substrate.

SUMMARY OF THE INVENTION

It has been discovered that bubbles of a fiber-cleansing gas (“scrubbinggas”) flowing parallel to fibers in a vertical skein are more effectivethan bubbles which are intercepted by arcuate fibers crossing the pathof the rising bubbles. Bubbles of an oxygen-containing gas to promotegrowth of microbes unexpectedly fails to build-up growth of microbes onthe surfaces of the fibers because the surfaces are “verticallyair-scrubbed”. Deposits of animate and/or inanimate particles upon thesurfaces of fibers are minimized when the restrictedly swayable fibersare kept awash in codirectionally rising bubbles which rise withsufficient velocity to exert a physical scrubbing force (momentumprovides the energy) to keep the fibers substantially free ofdeleterious deposits. Thus, an unexpectedly high flux is maintained overa long period during which permeate is produced by outside-in flowthrough the fibers.

It has also been discovered that permeate may be efficiently withdrawnfrom a substrate for a surprisingly long period, in a single stage,essentially continuous filtration process, by mounting a pair of headersin vertically spaced apart relationship, one above another, within thesubstrate which directly contacts a multiplicity of long vertical fibersin a “gas-scrubbed assembly” comprising a skein and a gas-distributionmeans. The skein has a surface area which is at least >1 m², and opposedspaced-apart ends of the fibers are secured in spaced-apart headers, sothat the fibers, when deployed in the substrate, acquire a generallyvertical profile therewithin and sway within the bubble zone defined byat least one column of bubbles. The length of fibers between opposedsurfaces of headers from which they extend, is in a critical range fromat least 0.1% (per cent) longer than the distance separating thoseopposed faces, but less than 5% longer. Usually the length of fibers isless than 2% longer, and most typically, less than 1% longer, so thatsway of the fibers is confined within a vertical zone of movement, theperiphery of which zone is defined by side-to-side movement of outerfibers in the skein; and, the majority of the fibers near the peripherymove in a slightly larger zone than one defined by the projected area ofone header upon the other. Though the distance between headers is fixedduring operation, the distance is preferably adjustable to provide anoptimum length of fibers, within the aforesaid ranges, between theheaders. It has been found that for no known reason, fibers which aremore than 5% but less than 10% longer than the fixed distance betweenthe opposed faces of the headers of a skein, tend to shear off at theface; and those 10% longer tend to clump up in the bubble zone.

The terminal end portions of the fibers are secured non-contiguously ineach header, that is, the surface of each fiber is sealingly separatedfrom that of another adjacent fiber with cured potting resin.Preferably, for maximum utilization of space on a header, the fibers aredeliberately set in a geometrically regular pattern. Typically permeateis withdrawn from the open ends of fibers which protrude from thepermeate-discharging aft (upper) face of a header. The overall geometryof potted fibers is determined by a ‘fiber-setting form’ used to setindividual fibers in an array. The skein operates in a substrate held ina reservoir at a pressure in the range from 1 atm to an elevatedpressure up to about 10 atm in a pressurized vessel, without beingconfined within the shell of a module.

It is therefore a general object of this invention to provide a novel,economical and surprisingly trouble-free membrane device, for providingan alternative to both, a conventional module having plural individualarrays therewithin, and also to a frameless array of arcuate fibers; thenovel device includes, (i) a vertical skein of a multiplicity ofrestrictedly swayable fibers, together having a surface area in therange from 1 m² to 1000 m², preferably from 10 m² to 100 m², securedonly in spaced-apart headers; and (ii) a gas-scrubbing means whichproduces at least one column of bubbles engulfing the skein. A skeinincludes permeate pans disposed, preferably non-removably, within asubstrate held in a reservoir of arbitrary proportions, the reservoirtypically having a volume in excess of 100 L (liters), generally inexcess of 1000 L. A fluid component is to be selectively removed fromthe substrate.

It is a specific object of this invention to provide a membrane devicehaving hollow fibers for removing permeate from a substrate, comprising,a skein of a multiplicity of fibers restrictedly swayable in thesubstrate, the opposed terminal end portions of which fibers inspaced-apart relationship, are potted in a pair of headers, one upperand one lower, each adapted to be mounted in vertically spaced apartgenerally parallel relationship, one above the other, within thesubstrate; essentially all the ends of fibers in both headers are openso as to pass permeate through the headers; the fibers in a skein have alength in the range from at least 0.1% greater, but less than 5% greaterthan the direct distance between opposed faces of the upper and lowerheaders, so as to present the fibers, when they are deployed, in anessentially vertical configuration; permeate is collected in acollection means, such as a permeate pan; and, permeate is withdrawnthrough a ducting means including one or more conduits and appropriatevalves.

It has also been discovered that skein fibers are maintainedsufficiently free from particulate deposits with surprisingly littlecleansing gas, so that the specific flux at equilibrium is maintainedover a long period, typically from 50 hr to 1500 hr, because the skeinis immersed so as to present a generally vertical profile, and, theskein is maintained awash in bubbles either continuously orintermittently generated by a gas-distribution means (“air-manifold”).The air-manifold is disposed adjacent the skein's lower header togenerate a column of rising bubbles within which column the fibers areawash in bubbles. A bank of skeins is “gas-scrubbed” with pluralair-tubes disposed between the lower headers of adjacent skeins, mostpreferably, also adjacent the outermost array of the first and lastskeins, so that for “n” headers there are “n+1” air-manifolds. Eachheader is preferably in the shape of a rectangular parallelpiped, theupper and lower headers having the same transverse (y-axis) dimension,so that plural headers are longitudinally stackable (along the x-axis).Common longitudinally positioned linear air-tubes, or, individual,longitudinally spaced apart vertically rising air-tubes, service thebank and one or more permeate tubes withdraw permeate.

It is therefore a general object of this invention to provide agas-scrubbed assembly of fibers for liquid filtration, the assemblycomprising, (a) bank of gas-scrubbed skeins of fibers which separate adesired permeate from a large body of multicomponent substrate havingfinely divided particulate matter in the range from 0.1 μm-44 μmdispersed therein, (b) each skein comprising at least 20 fibers havingupper and lower terminal portions potted spaced-apart in upper and lowerheaders, respectively, the fibers being restrictedly swayable in abubble zone, and (c) a shaped gas-distribution means adapted to providea profusion of vertically ascending bubbles near the lower header, thelength of the fibers being from at least 0.1% but less than 5% greaterthan the distance between the opposed faces of the headers. Thegas-distribution means has through-passages therein through which gas isflowed at a flow rate which is proportional to the number of fibers. Theflow rate is generally in the range from 0.47-14 cm³/sec per fiber(0.001-0.03 scfm/fiber) (standard ft³ per minute per fiber), typicallyin the range from 1.4-4.2 cm³/sec/fiber (0.003-0.009 scfm/fiber). Thesurface area of the fibers is not used to define the amount of air usedbecause the air travels substantially vertically along the length ofeach fiber. The gas generates bubbles having an average diameter in therange from about 0.1 mm to about 25 mm, or even larger.

It is a specific object of this invention to provide the aforesaid novelgas-scrubbed assembly comprising, a bank of vertical skeins and a shapedgas-distribution means for use with the bank, in a substrate in whichmicroorganisms grow, the assembly being used in combination withvertically adjustable spacer means for mounting the headers invertically spaced apart relationship, and in open fluid communicationwith collection means for collecting the permeate; means for withdrawingthe permeate; and, sufficient air is flowed through the shapedgas-distribution means to generate enough bubbles flowing upwardlythrough the skein, between and parallel to the fibers so as to keep thesurfaces of the fibers substantially free from deposits of livemicroorganisms as well as small inanimate particles which may be presentin the substrate.

It has still further been discovered that a system utilizing a bank ofvertical skeins of fibers potted in headers vertically spaced-apart byspacer means, and deployed in a substrate containing particulatematerial, in combination with a proximately disposed gas-distributionmeans to minimize fouling of the membranes, may be operated to withdrawpermeate under gravity alone, so that the cost of any pump to withdrawpermeate is avoided, provided the net positive suction headcorresponding to the vertical height between the level of substrate, andthe location of withdrawal of permeate, provides the transmembranepressure differential under which the fibers function in the skein

It is therefore a general object of this invention to provide theforegoing system in which opposed terminal end portions of skein fibersart essentially free from fiber-to-fiber contact after being potted inupper and lower headers kept vertically spaced-apart with spacer means,the skein being unconfined in a shell of a module and deployed in thesubstrate without the fibers being supported during operation except bythe spacer means which support only the headers; the headers beingmounted so that the fibers present a generally vertical profile yet arerestrictedly swayable in a zone of confinement defined by risingbubbles; means for mounting each header in open fluid communication withcollection means for collecting permeate, and, means for withdrawing thepermeate; and, shaped gas-distribution means adapted to generate bubblesfrom micron-size to 25 mm in nominal diameter, most preferably in thesize range from 1 mm to 20 mm, the bubbles flowing upwardly through andparallel to the fibers at a flow rate chosen from the range specifiedhereabove; whereby the fibers are scrubbed with bubbles and resist theattachment of growing microorganisms and any other particulate matter tothe surfaces of the fibers, so as to maintain a desirable specific fluxduring operation.

Still further, a low cost process has been discovered for treating amulticomponent substrate under pressure ranging from 1-10 atm in apressurizable vessel, particularly for example, an aqueous streamcontaining finely divided inorganic matter such as silica, silicic acid,or, activated sludge, when the substrate is confined in a large tank orpond, by using a bank of vertical skeins each comprising restrictedlyswayable unsupported fibers potted in headers in open fluidcommunication with a means for withdrawing permeate, in combination witha source of air which generates bubbles near the lower header.

It is therefore a general object of this invention to provide a processfor maintaining relatively clean fiber surfaces in an array of amembrane device while separating a permeate from a substrate, theprocess comprising, submerging a skein of restrictedly swayablesubstantially vertical fibers within the substrate so that upper andlower headers of the skein are mounted one above the other with amultiplicity of fibers secured between said headers, the fibers havingtheir opposed terminal portions in open fluid communication withpermeate collecting means in fluid-tight connection with said headers;the fibers operating under a transmembrane pressure differential in therange from about 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), and alength from at least 0.1% to about 2% greater than the direct distancebetween the opposed faces of upper and lower headers, so as to present,when the fibers are deployed, a generally vertical skein of fibers;

maintaining an essentially constant flux substantially the same as theequilibrium flux initially obtained, indicating that the surfaces of thefibers are substantially free from further build-up of deposits once theequilibrium flux is attained; collecting the permeate; and,

withdrawing the permeate.

It has still further been discovered that the foregoing process may beused in the operation of an anaerobic or aerobic biological reactorwhich has been retrofitted with the membrane device of this invention.The anaerobic reactor is a closed vessel and the scrubbing gas is amolecular oxygen-free gas, such as nitrogen.

It is therefore a general object of this invention to provide an aerobicbiological reactor retrofitted with at least one gas-scrubbed bank ofvertical skeins, each skein made with from 500 to 5000 fibers in therange from 1 m to 3 m long, in combination with a permeate collectionmeans, and to provide a process for the reactor's operation withoutbeing encumbered by the numerous restrictions and limitations imposed bya secondary clarification system.

A novel composite header is provided for a bundle of hollow fibermembranes or “fibers”, the composite header comprising a molded,laminated body of arbitrary shape, having an upper lamina formed from a“fixing” (potting) material which is laminated to a lower lamina formedfrom a “fugitive” potting material. The terminal portions of the fibersare potted in the fugitive potting material when it is liquid,preferably forming a generally rectangular parallel-piped in which theopen ends of the fibers (until potted) are embedded and plugged, keepingthe fibers in closely spaced-apart substantially parallel relationship.The plugged ends of the fibers fail to protrude through the lower (aft)face of the lower lamina, while the remaining lengths of the fibersextend through the upper face of the lower lamina. The upper laminaextends for a height along the length of the fibers sufficient tomaintain the fibers in the same spaced-apart relationship relative toone and another as their spaced-apart relationship in the lower portion.If desired, the composite header may include additional laminae, forexample, a “cushioning” lamina overlying the fixing lamina, to cushioneach fiber around its embedded outer circumference; and, a “gasketing”lamina to provide a suitable gasketing material against which thepermeate collection means may be mounted.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional objects and advantages of the inventionwill best be understood by reference to the following detaileddescription, accompanied by schematic illustrations of preferredembodiments of the invention, in which illustrations like referencenumerals refer to like elements, and in which:

FIG. 1 is a graph in which the variation of flux is plotted as afunction of time, showing three curves for three runs made with threedifferent arrays, in each case, using the same amount of air, theidentical membranes and the same membrane surface area The resultsobtained by Yamamoto et al are plotted as curve 2 (under conditionsmodified to give them the benefit of doubt as to the experimentalprocedure employed, as explained below); the flux obtained using thegas-scrubbed assembly of the '424 patent is shown as curve 1; and theflux obtained using the gas-scrubbed assembly of this invention is shownas curve 3.

FIG. 2 is a perspective exploded view schematically illustrating amembrane device comprising a skein of fibers, unsupported duringoperation of the device, with the ends of the fibers potted in a lowerheader, along with a permeate collection pan, and a permeate withdrawalconduit. By “unsupported” is meant ‘not supported except for spacermeans to space the headers’.

FIG. 2A is an enlarged detail side elevational view of a side wall of acollection pan showing the profile of a header-retaining step atop theperiphery of the pan.

FIG. 2B is a bottom plan view of the header showing a random pattern ofopen ends protruding from the aft face of a header when fibers arepotted after they are stacked in rows and glued together before beingpotted.

FIG. 3 is a perspective view of a single array, schematicallyillustrated, of a row of substantially coplanarly disposed parallelfibers secured near their opposed terminal ends between spaced apartcards. Typically, multiple arrays are assembled before beingsequentially potted.

FIG. 4 illustrates a side elevational view of a stack of arrays near oneend where it is together, showing that the individual fibers (only thelast fiber of each linear array is visible, the remaining fibers in thearray being directly behind the last fiber) of each array are separatedby the thickness of a strip with adhesive on it, as the stack is heldvertically in potting liquid.

FIG. 5 is a perspective view schematically illustrating a skein with itsintegral finished header, its permeate collection pan, and twinair-tubes feeding an integral air distribution manifold potted in theheader along an outer edge of the skein fibers.

FIG. 6 is a side elevational view of an integral finished header showingdetails of a permeate pan submerged in substrate, the walls of theheader resting on the bottom of a reservoir, and multiple air-tubesfeeding integral air distribution manifolds potted in the header alongeach outer edge of the skein fibers.

FIG. 7A is a perspective view schematically illustrating an air-manifoldfrom which vertical air-tubes rise.

FIG. 7B is a perspective view schematically illustrating a tubularair-manifold having a transverse perforated portion, positioned byopposed terminal portions.

FIG. 8 is a perspective view of an integral finished header havingplural skeins potted in a common header molded in an integral permeatecollection means with air-tubes rising vertically through the headerbetween adjacent skeins, and along the outer peripheries of the outerskeins.

FIG. 9 is a detail, not to scale, illustratively showing a gasdistribution means discharging gas between arrays in a header, andoptionally along the sides of the lower header.

FIG. 10 is a perspective view schematically illustrating a pair ofskeins in a bank in which the upper headers are mounted by their ends onthe vertical wall of a tank. The skeins in combination with agas-distribution means form a “gas-scrubbing assembly” deployed within asubstrate, with the fibers suspended essentially vertically in thesubstrate. Positioning the gas-distribution means between the lowerheaders (and optionally, on the outside of skein fibers) generate masses(or “columns”) of bubbles which. rise vertically, codirectionally withthe fibers, yet the bubbles scrub the outer surfaces of the fibers.

FIG. 11 is a perspective view of another embodiment of thescrubbing-assembly showing plural skeins (only a pair is shown)connected in a bank with gas-distribution means disposed betweensuccessive skeins, and, optionally, with additional gas-distributionmeans fore and aft the first and last skeins, respectively.

FIG. 12 is an elevational view schematically illustrating a bank ofskeins mounted against the wall of a bioreactor, showing the convenienceof having all piping connections outside the liquid.

FIG. 13 is a plan view of the bioreactor shown in FIG. 12 showing howmultiple banks of skeins may be positioned around the circumference ofthe bioreactor to form a large permeate extraction zone while aclarification zone is formed in the central portion with the help ofbaffles.

FIG. 14 illustratively shows another embodiment of the skein in whichthe permeate tube is concentrically disposed within the air supply tubeand both are potted, near their lower ends in the lower header. Ports inthe lower end of the air supply tube provide air near the base of theskein fibres.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The skein of this invention may be used in a liquid-liquid separationprocess of choice, and more generally, in various separation processes.The skein is specifically adapted for use in Microfiltration processesused to remove large organic molecules, emulsified organic liquids andcolloidal or suspended solids, usually from water. Typical applicationsare (i) in a membrane bioreactor, to produce permeate as purified waterand recycle biomass; for (ii) tertiary filtration of wastewater toremove suspended solids and pathogenic bacteria; (iii) clarification ofaqueous streams including filtration of surface water to producedrinking water (removal of colloids, long chain carboxylic acids andpathogens); (iv) separation of a permeable liquid component inbiotechnology broths; (v) dewatering watering of metal hydroxidesludges; and, (vi) filtration of oily wastewater, inter alia.

The problem with using a conventional membrane module to selectivelyseparate one fluid from another, particularly using the module incombination with a bioreactor, and the attendant costs of operating sucha system, have been avoided. In those instances where an under-developedcountry or distressed community lacks the resources to provide membranemodules, the most preferred embodiment of this invention is adapted foruse without. any pumps. In those instances where a pump is convenientlyused, a vacuum pump is unnecessary, adequate driving force beingprovided by a simple centrifugal pump incapable of inducing a vacuum of75 cm Hg on the suction side.

The fibers used to form the skein may be formed of any conventionalmembrane material provided the fibers are flexible and have an averagepore cross sectional diameter for microfiltration, namely in the rangefrom about 1000 Å to 10000 Å. Preferred fibers operate with atransmembrane pressure differential in the range from 7 kPa (1 psi)-69kPa (10 psi) and are used under ambient pressure with the permeatewithdrawn under gravity. The fibers are chosen with a view to performtheir desired function, and the dimensions of the skein are determinedby the geometry of the headers and length of the fibers. It isunnecessary to confine a skein in a modular shell, and a skein is not.

Preferred fibers are made of organic polymers and ceramics, whetherisotropic, or anisotropic, with a thin layer or “skin” on the outsidesurface of the fibers. Some fibers may be made from braided cottoncovered with a porous natural rubber latex or a water-insolublecellulosic polymeric material. Preferred organic polymers for fibers arepolysulfones, poly(styrenes), including styrene-containing copolymerssuch as acrylonitrile-styrene, butadiene-styrene andstyrene-vinylbenzylhalide copolymers, polycarbonates, cellulosicpolymers, polypropylene, poly(vinyl chloride), poly(ethyleneterephthalate), and the like disclosed in U.S. Pat. No. 4,230,463 thedisclosure of which is incorporated by reference thereto as if fully setforth herein. Preferred ceramic fibers are made from alumina, by E. I.duPont deNemours Co. and disclosed in U.S. Pat. No. 4,069,157.

Typically, there is no cross flow of substrate across the surface of thefibers in a “dead end” tank. If there is any flow of substrate throughthe skein in a dead end tank, the flow is due to aeration providedbeneath the skein, or to such mechanical mixing as may be employed tomaintain the solids in suspension. There is more flow through the skeinin a tank into which substrate is being continuously flowed, but thevelocity of fluid across the fibers is generally too insignificant todeter growing microorganisms from attaching themselves, or suspendedparticles, e.g. microscopic siliceous particles, from being deposited onthe surfaces of the fibers.

For hollow fiber membranes, the outside diameter of a fiber is at least20 μm and may be as large as about 3 mm, typically being in the rangefrom about 0.1 mm to 2 mm. The larger the outside diameter the lessdesirable the ratio of surface area per unit volume of fiber. The wallthickness of a fiber is at least 5 μm and may be as much as 1.2 mm,typically being in the range from about 15% to about 60% of the outsidediameter of the fiber, most preferably from 0.5 mm to 1.2 mm.

As in a '424 array, but unlike in a conventional module, the length of afiber in a skein is essentially independent of the strength of thefiber, or its diameter, because the skein is buoyed both by bubbles andthe substrate in which it is deployed. The length of fibers in the skeinis preferably determined by the conditions under which the skein is tooperate. Typically fibers range from 1 m to about 5 m long, dependingupon the dimensions of the body of substrate (depth and width) in whichthe skein is deployed.

The fixing material to fix the fibers in a finished header is mostpreferably either a thermosetting or thermoplastic synthetic resinousmaterial, optionally reinforced with glass fibers, boron or graphitefibers and the like. Thermoplastic materials may be crystalline, such aspolyolefins, polyamides (nylon), polycarbonates and the like,semi-crystalline such as polyetherether ketone (PEEK), or substantiallyamorphous, such as poly(vinyl chloride) (PVC), polyurethane and thelike. Thermosetting resins commonly include polyesters, polyacetals,polyethers, cast acrylates, thermosetting polyurethanes and epoxyresins. Most preferred as a “fixing” material (so termed because itfixes the locations of the fibers relative to each other) is one whichwhen cured is substantially rigid in a thickness of about 2 cm, andreferred to generically as a “plastic” because of its hardness. Such aplastic has a hardness in the range from about Shore D 50 to Rockwell R110 and is selected from the group consisting of epoxy resins,phenolics, acrylics, polycarbonate, nylon, polystyrene, polypropyleneand ultra-high molecular weight polyethylene (UHMW PE). Polyurethanesuch as is commercially available under the brand names Adiprene® fromUniroyal Chemical Company and Airthane® from Air Products, andcommercially available epoxy resins such as Epon 828 are excellentfixing materials.

The number of fibers in an array is arbitrary, typically being in therange from about 1000 to about 10000 for commercial applications, andthe preferred surface area for a skein is in the range from 10 m² to 100m².

The particular method of securing the fibers in each of the headers isnot narrowly critical, the choice depending upon the materials of theheader and the fiber, and the cost of using a method other than potting.However, it is essential that each of the fibers be secured influid-tight relationship within each header to avoid contamination ofpermeate. This is effected by potting the fibers essentially vertically,in closely-spaced relationship, either linearly in plural equally spacedapart rows across the face of a header in the x-y plane; oralternatively, randomly, in non-linear plural rows. In the latter, thefibers are displaced relative to one another in the lateral direction.

FIG. 1 presents the results of a comparison of three runs made, oneusing the teachings of Yamamoto in his '89 publication (curve 2), butusing an aerator which introduced air from the side and directed itradially inwards, as is shown in Chiemchaisri et al. A second run (curve1) uses the gas-scrubbed assembly of the '424 patent, and the third run(curve 3) uses the gas-scrubbed skein of this invention. The specificflux obtained with an assembly of an inverted parabolic array with anair distributor means (Yamamoto et al), as disclosed in Wat. Sci. Tech.Vol. 21, Brighton pp 43-54, 1989, and, the parabolic array by Cote et alin the '424 patent, are compared to the specific flux obtained with thevertical skein of this invention.

The comparison is for the three assemblies having fibers with nominalpore size 0.2 μm with essentially identical bores and surface area in 80L tanks filled with the same activated sludge substrate. The differencesbetween the stated experiment of Yamamoto et al, and that of the '424patent are of record in the '424 patent, and the conditions of thecomparison are incorporated by reference thereto as if fully set forthherein. The vertical skein used herein differs from the '424 skein onlyin the vertical configuration of the 280 fibers each of which was about1% longer than the distance between the spaced apart headers duringoperation. The flow rate of air for the vertical skein is 1.4 m³/hr/m²using a coarse bubble diffuser.

It will be evident from FIG. 1 in which the specific flux, liters/meter²hr/kPa (conventionally written as (1 mh/kPa), is plotted as a functionof operating time for the three assemblies, that the curve, identifiedas reference numeral 3 for the flux for the vertical skein, providesabout the same specific flux as the parabolic skein, identified asreference numeral 1. As can be seen, each specific flux reaches anequilibrium condition within less than 50 hr, but after about 250 hr, itis seen that the specific flux for the inverted parabolic array keepsdeclining but the other two assemblies reach an equilibrium.

Referring to FIG. 2 there is illustrated, in exploded view a portion ofa membrane device referred to as a “vertical skein” 10, comprising alower header 11 of a pair of headers, the other upper header (not shown)being substantially identical; a collection pan 20 to collect thepermeate; and, a permeate withdrawal conduit 30. The header shown is arectangular prism since this is the most convenient shape to make, ifone is going to pot fibers 12 in a potting resin such as a polyurethaneor an epoxy. Though the fibers 12 are not shown as close together asthey would normally be, it is essential that the fibers are not incontact with each other but that they be spaced apart by the cured resinbetween them.

As illustrated, the open ends of the terminal portion 12′ of the fibersare in the same plane as the lower face of the header 11 because thefibers are conventionally potted and the header sectioned to expose theopen ends. A specific potting procedure in which the trough of aU-shaped bundle of fibers is potted, results in forming two headers.This procedure is described in the '424 patent (col 17, lines 44-61);however, even cutting the potted fibers with a thin, high-speed diamondblade, tends to damage the fibers and initiate the collapse of thecircumferential wall. In another conventional method of potting fibers,described in U.S. Pat. No. 5,202,023, bundled fibers have their endsdipped in resin or paint to prevent potting resin penetration into thebores of the fibers during the potting process. The ends of the bundleare then placed in molds and 10 uncured resin added to saturate the endsof the fiber bundle and fill the spaces between the individual fibers inthe bundle and the flexible tubing in which the bundle is held. Thecured molded ends are removed from the molds and the molded ends cut off(see, bridging cols 11 and 12). In each art method, sectioning the molddamages the embedded fibers.

Therefore a novel method is used to form a header 11 in the form of arectangular prism. The method requires forming a composite header withtwo liquids. A first liquid fugitive material, when solidified (cured),forms a “fugitive lamina” of the composite header; a second liquid ofnon-fugitive fixing material forms a “fixing lamina”. By a “fugitivematerial” we refer to a material which is either (i) soluble in a mediumin which the fibers and fixing material are not soluble, or (ii)fluidizable by virtue of having a melting point (if the material iscrystalline) below that which might damage the fibers or fixingmaterial; or, the material has a glass transition temperature Tg (if thematerial is non-crystalline), below that which might damage the fibersor material(s) forming the non-fugitive header; or (iii) both solubleand fluidizable.

The first liquid is poured around terminal portions of fibers, allowedto cool and solidify into a fugitive lamina; the fibers in the fugitivelamina are then again potted, this time by pouring the second liquidover the solid fugitive lamina.

In greater detail, the method for forming a finished header for skeinfibers comprises,

forming a stack of at least two superimposed essentially coplanar andsimilar arrays, each array comprising a chosen number of fiberssupported on a support means having a thickness corresponding to adesired lateral spacing between adjacent arrays;

holding the stack in a first liquid with terminal portions of the fiberssubmerged, until the liquid solidifies into a first shaped lamina,provided that the first liquid is unreactive with material of thefibers;

pouring a second liquid over the first shaped lamina to embed the fibersto a desired depth, and solidifying the second liquid to form a fixinglamina upon the first shaped lamina, the second liquid also beingsubstantially unreactive with either the material of the fibers or thatof the first shaped lamina;

whereby a composite header is formed in which terminal portions of thefibers are potted, preferably in a geometrically regular pattern, thecomposite header comprising a laminate of a fugitive lamina of fugitivematerial and a contiguous finished header of fixing lamina; andthereafter,

removing the first shaped lamina without removing a portion of thefixing lamina so as to leave the ends of the fibers open and protrudingfrom the aft face of the header, the open ends having circularcross-section.

The step-wise procedure for forming an array “A” with the novel headeris described with respect to an array illustrated in FIG. 3, as follows:

A desired number of fibers 12 are each cut to about the same length witha sharp blade so as to leave both opposed ends of each fiber with anessentially circular cross-section. The fibers are coplanarly disposedside-by-side in a linear array on a planar support means such as stripsor cards 15 and 16. Preferably the strips are coated with an adhesive,e.g. a commercially available polyethylene hot-melt adhesive, so thatthe fibers are glued to the strips and opposed terminal portions 12″respectively of the fibers, extend beyond the strips. Intermediateportions 12′ of the fibers are thus secured on the strips.Alternatively, the strips may be grooved with parallel spaced-apartgrooves which snugly accommodate the fibers. The strips may be flexibleor rigid. If flexible, strips with fibers adhered thereto, are in turn,also adhered to each other successively so as to form a progressivelystiffer stack for a header having a desired geometry of potted fibers.To avoid gluing the strips, a regular pattern of linear rows may beobtained by securing multiple arrays on rigid strips in a stack, withrubber bands 18 or other clamping means. The terminal portions 12″ arethus held in spaced-apart relationship, with the center to centerdistance of adjacent fibers preferably in the range from 1.2 (1.2 d) toabout 5 times (5 d) the outside diameter ‘d’ of a fiber. Spacing thefibers further apart wastes space and spacing them closer increases therisk of fiber-to-fiber contact near the terminal end portions when theends are potted. Preferred center-to-center spacing is from about 1.5 dto 2 d. The thickness of a strip and/or adhesive is sufficient to ensurethat the fibers are kept spaced apart. Preferably, the thickness isabout the same as, or relatively smaller than the outside diameter of afiber, preferably from about 0.5 d to 1 d thick, which becomes thespacing between adjacent outside surfaces of fibers in successive lineararrays.

Having formed a first array, a second array (not shown because it wouldappear essentially identical to the first) is prepared in a manneranalogous to the first, strip 15 of the second array is overlaid uponthe intermediate portions 12′ on strip 15 of the first array, the strip15 of the second array resting on the upper surfaces of the fiberssecured in strip 15 of the first array. Similarly, strip 16 of thesecond array is overlaid upon the intermediate portions 12′ on strip 16of the first array.

A third array (essentially identical to the first and second) isprepared in a manner analogous to the first, and then overlaid upon thesecond, with the strips of the third array resting on the upper surfacesof the fibers of the second array.

Additional arrays are overlaid until the desired number of arrays arestacked in rows forming a stack of arrays with the adhesive-coatedstrips forming the spacing means between successive rows of fibers. Thestack of arrays on strips is then held vertically to present the lowerportion of the stack to be potted first.

Referring to FIG. 4, there is schematically illustrated a rectangularpotting pan 17 the length and width dimensions of which correspondsubstantially to the longitudinal (x-axis) and transverse (y-axis)dimensions respectively, of the desired header. The lower stack issubmerged in a first liquid which rises to a level indicated by L1, inthe pan 17. Most preferred is a liquid wax, preferably a water-solublewax having a melting point lower than 75° C., such as a polyethyleneglycol (PEG) wax.

The depth to which the first liquid is poured will depend upon whetherthe strips 15 are to be removed from, or left in the finished header.

A. First illustrated is the potting of skein fibers in upper and lowerheaders from which the strips will be removed.

(1) A first shaped lamina having a thickness L1 (corresponding tothe-depth to which the first liquid was poured) is formed to provide afugitive lamina from about 5-10 cm thick. The depth of the first liquidis sufficient to ensure that both the intermediate portions 12′ on thestrips and terminal portions 12″ will be held spaced apart when thefirst liquid solidifies and plugs all the fibers.

(2) The second liquid, a curable, water-insoluble liquid potting resin,or reactive components thereof, is poured over the surface of thefugitive lamina to surround the fibers, until the second liquid rises toa level L2. It is solidified to form the fixing lamina (which will bethe finished header) having a thickness measured from the level L1 tothe level L2 (the thickness is written “L1-L2”). The thickness L1-L2 ofthe fixing lamina, typically from about 1 cm to about 5 cm, issufficient to maintain the relative positions of the vertical fibers. Afirst composite header is thus formed having the combined thicknesses ofthe fugitive and fixing laminae.

(3) In a manner analogous to that described immediately hereinabove, astack is potted in a second composite header.

(4) The composite headers are demolded from their potting pans and hotair blown over them to melt the fugitive laminae, leaving only thefinished headers, each having a thickness L1-L2. The fugitive materialsuch as the PEG wax, is then reused. Alternatively, a water-solublefugitive material may be placed in hot water to dissolve the wax, andthe material recovered from its water solution.

(5) The adhered strips and terminal portions of the fibers which wereembedded within the fugitive lamina are left protruding from thepermeate-discharging aft faces of the headers with the ends of thefibers being not only open, but essentially circular in cross section.The fibers may now be cut above the strips to discard them and theterminal portions of the fibers adhered to them yet maintaining thecircular open ends. The packing density of fibers, that is, the numberof fibers per unit area of header preferably ranges from 4 to 50fibers/cm² depending upon the diameters of the fibers.

B. Illustrated second is the potting of skein fibers in upper and lowerheaders from which the strips will not be removed, to avoid the step ofcutting the fibers.

(1) The first liquid is poured to a level L1′ below the cards, to adepth in the range from about 1-2.5 cm, and solidified, forimingfugitive lamina L1′.

(2) The second liquid is then poured over the fugitive lamina to depthL2 and solidified, forming a composite header with a fixing laminahaving a thickness L1′-L2.

(3) The composite header is demolded and the fugitive lamina removed,leaving the terminal portions 12″ protruding from the aft face of thefinished header, which aft face is formed at what had been the levelL1′. The finished header having a thickness L1′-L2 embeds the strips 15(along with the rubber bands 18, if used).

C. Illustrated third is the potting of skein fibers to form a finishedheaders with a cushioning lamina embedding the fibers on the opposed(fore) faces of the headers from which the strips will be removed.

The restricted swayability of the fibers generates some intermittent‘snapping’ motion of the fibers. This motion has been found to break thepotted fibers around their circumferences, at the interface of the foreface and substrate. The hardness of the fixing material which forms a“fixing lamina” was found to initiate excessive shearing forces at thecircumference of the fiber. The deleterious effects of such forces isminimized by providing a cushioning lamina of material softer than thefixing lamina. Such a cushioning lamina is formed integrally with thefixing lamina, by pouring cushioning liquid (so termed for its functionwhen cured) over the fixing lamina to a depth L3 as shown in FIG. 4,which depth is sufficient to provide enough ‘give’ around thecircumferences of the fibers to minimize the risk of shearing. Suchcushioning liquid, when cured is rubbery, having a hardness in the rangefrom about Shore A 30 to Shore D 45, and is preferably a polyurethane orsilicone or other rubbery material which will adhere to the fixinglamina. Upon removal of the fugitive lamina, the finished header thusformed has the combined thicknesses of the fixing lamina and thecushioning lamina, namely L1-L3 when the strips 15 are cut away.

D. Illustrated fourth is the formation a finished header with agasketing lamina embedding the fibers on the header's aft face, and acushioning lamina embedding the fibers on the header's fore face; thestrips are to be removed.

Whichever finished header is made, it is preferably fitted into apermeate pan 20 as illustrated in FIG. 2 with a peripheral gasket. Ithas been found that it is easier to seal the pan against a gasketinglamina, than against a peripheral narrow gasket. A relatively softgasketing material having a hardness in the range from Shore A 40 toShore D 45, is desirable to form a gasketing lamina integrally with theaft face of the finished header. In the embodiment in which the stripsare cut away, the fugitive lamina is formed as before, and a gasketingliquid (so termed because it forms the gasket when cured) is poured overthe surface of the fugitive lamina to a depth L4. The gasketing liquidis then cured. Upon removal of the fugitive lamina, when the strips 15are cut away, the finished header thus formed has the combinedthicknesses of the gasketing lamina (L1-L4), the fixing lamina (L4-L2)and the cushioning lamina (L2-L3), namely an overall L1-L3.

In another embodiment, to avoid securing the pan to the header with agasketing means, and, to avoid positioning one or more gas-distributionmanifolds in an optimum location near the base of the skein fibers aftera skein is made, the manifolds are formed integrally with a header.Referring to FIG. 5 there is illustrated in perspective view an“integral single skein” referred to generally by reference numeral 100.The integral single skein is so termed because it includes an integralfinished header 101 and permeate pan 102. The pan 102 is provided with apermeate withdrawal nipple 106, and fitted with vertical air-tubes 103which are to be embedded in the finished header. The air-tubes arepreferably manifolded on either side of the skein fibers, to feederair-tubes 104 and 105 which are snugly inserted through grommets in thewalls of the pan. The permeate nipple 106 is then plugged, and a stackof arrays is held vertically in the pan in which a fugitive lamina isformed embedding both the ends of the fibers and the lower portion ofthe vertical air-tubes 103. A fixing lamina is then formed over thefugitive lamina, embedding the fibers to form a fixing lamina throughwhich protrude the open ends of the air-tubes 103. The fugitive laminais then melted and withdrawn through the nipple 106. In operation,permeate collects in the permeate pan and is withdrawn through nipple106.

FIG. 6 illustrates a cross-section of an integral single skein 110 withanother integral finished header 101 having a thickness L1-L2, butwithout a cushioning lamina, formed in a procedure similar to thatdescribed hereinabove. A permeate pan 120 with outwardly flared sides120′ and transversely spaced-apart through-apertures therein, isprefabricated between side walls 111 and 112 so the pan is spaced abovethe bottom of the reservoir.

A pair of air-manifolds 107 such as shown in FIGS. 7A or 7B, ispositioned and held in mirror-image relationship with each otheradjacent the permeate pan 120, with the vertical air-tubes 103protruding through the apertures in sides 120′, and the ends 104 and 105protrude from through-passages in the vertical walls on either side ofthe permeate pan. Permeate withdrawal nipple 106 (FIG. 6) is firsttemporarily plugged. The stack of strips 15 is positioned betweenair-tubes 103, vertically in the pan 120 which is filled to level L1 toform a fugitive lamina, the level being just beneath the lower edges ofthe strips 15 which will not be removed. When solidified, the fugitivelamina embeds the terminal portions of the fibers 12 and also fillspermeate tube 106. Then the second liquid is poured over the uppersurface of the fugitive lamina until the liquid covers the strips 15 butleaves the upper ends of the air-tubes 103 open. The second liquid isthen cured to form the fixing lamina of the composite header which isthen heated to remove the fugitive material through the permeate nozzle106 after it is unplugged.

FIG. 7A schematically shows in perspective view, an air-manifold 107having vertical air-tubes 103 rising from a transverse header-tube whichhas longitudinally projecting feeder air-tubes 104 and 105. The bore ofthe air-tubes which may be either “fine bubble diffusers”, or “coarsebubble diffusers”, or “aerators”, is chosen to provide bubbles of thedesired diameter under operating conditions, the bore typically being inthe range from 0.1 mm to 5 mm Bubbles of smaller diameter are preferablyprovided with a perforated transverse tube 103′ of an air-manifold 107′having feeder air-tubes 104′ and 105′, illustrated in FIG. 7B. In eachcase, the bubbles function as a mechanical brush.

The skein fibers for the upper header of the skein are potted in amanner analogous to that described above in a similar permeate pan toform a finished header, except that no air manifolds are inserted.

Referring to FIG. 8 there is schematically illustrated, in across-sectional perspective view, an embodiment in which a bank of twoskeins is potted in a single integral finished header enclosure,referred to generally by reference numeral 120 b. The term “headerenclosure” is used because its side walls 121 and 122, and end walls(not shown) enclose a plenum in which air is introduced. Instead of apermeate pan, permeate is collected from a permeate manifold whichserves both skeins. Another similar upper enclosure 120 u (not shown),except that it is a flat-bottomed channel-shaped pan (inverted for useas the upper header) with no air-tubes molded in it, has the opposedterminal portions of all the skein fibers potted in the pan. Foroperation, both the lower and upper enclosures 120 b and 120 u, withtheir skein fibers are lowered into a reservoir of the substrate to befiltered. The side walls 121 and 122 need not rest on the bottom of thereservoir, but may be mounted on a side wall of the reservoir.

The side walls 121 and 122 and end walls are part of an integrallymolded assembly having a platform 123 connecting the walls, and thereare aligned multiple risers 124 molded into the platform. The risersresemble an inverted test-tube, the diameter of which need only be largeenough to have an air-tube 127 inserted through the top 125 of theinverted test-tube. As illustrated, it is preferred to have “n+1” rowsof air-tubes for “n” stacks of arrays to be potted. Crenelated platform123 includes risers 124 between which lie channels 128 and 129. Channels128 and 129 are each wide enough to accept a stack of arrays of fibers12, and the risers are wide enough to have air-tubes 127 of sufficientlength inserted tberethrough so that the upper open ends 133 of theair-tubes protrude from the upper surface of the fixing material 101.The lower ends 134 of the air-tubes are sectioned at an angle tominimize plugging, and positioned above the surface S of the substrate.The channel 129 is formed so as to provide a permeate withdrawal tube126 integrally formed with the platform 123. Side wall 122 is providedwith an air-nipple 130 through which air is introduced into the plenumformed by the walls of the enclosure 120 b, and the surface S ofsubstrate under the platform 123. Each stack is potted as described inrelation to FIG. 6 above, most preferably by forming a composite headerof fugitive PEG wax and epoxy resin around the stacks of arrayspositioned between the rows of risers 124, making sure the open ends ofthe air-tubes are above the epoxy fixing material, and melting out thewax through the permeate withdrawal tube 126. When air is introducedinto the enclosure the air will be distributed through the air-tubesbetween and around the skeins.

Referring to FIG. 9 there is shown a schematic illustration of a skeinhaving upper and lower headers 41 u and 41 b respectively, and in each,the protruding upper and lower ends 12 u″ and 12 b″ are evidence thatthe face of the header was not cut to expose the fibers. The height ofthe contiguous intermediate portions 12 u′ and 12 b′ respectively,corresponds to the cured depth of the fixing material.

It will now be evident that the essential feature of the foregoingpotting method is that a fugitive lamina is formed which embeds theopenings of the terminal portions of the fibers before their contiguousintermediate portions 12 u′ and 12 u″ and 12 b′ and 12 b″ respectivelyare fixed in a fixing lamina of the header. An alternative choice ofmaterials is the use of a fugitive potting compound which is soluble ina non-aqueous liquid in which the fixing material is not soluble. Stillanother choice is to use a water-insoluble fugitive material which isalso insoluble in non-aqueous liquids typically used as solvents, butwhich fugitive material has a lower melting point than the final pottingmaterial which may or may not be water-soluble.

The fugitive material is inert relative to both, the material of thefibers as well as the final potting material to be cast, and thefugitive material and fixing material are mutually insoluble. Preferablythe fugitive material forms a substantially smooth-surfaced solid, butit is critical that the fugitive material be at least partially cured,sufficiently to maintain the shape of the header, and remain a solidabove a temperature at which the fixing material is introduced into theheader mold. The fugitive lamina is essentially inert and insoluble inthe final potting material, so that the fugitive lamina is removablyadhered to the fixing lamina.

The demolded header is either heated or solvent extracted to remove thefugitive lamina. Typically, the fixing material is cured to a firm solidmass at a first curing temperature no higher than the melting point orTg of the fugitive lamina, and preferably at a temperature lower thanabout 60° C.; the firm solid is then post-cured at a temperature highenough to melt the fugitive material but not high enough to adverselyaffect the curing of the fixing material or the properties of thefibers. The fugitive material is removed as described hereinafter, themethod of removal depending upon the fugitive material and the curingtemperature of the final potting material used.

Since, during operation, a high flux is normally maintained if cleansingair contacts substantially all the fibers, it will be evident that whenit is desirable to have a skein having a cross-section which is otherthan generally rectangular, for example elliptical or circular, orhaving a geometrically irregular periphery, and it is desired to have alarge number of skein fibers, it will be evident that the procedure forstacking consecutive peripheral arrays described above will be modified.Further, the transverse central air-tube 52 (see FIG. 9) is found to beless effective in skeins of non-rectangular cross-section than avertical air-tube which discharges air radially along its verticallength and which vertical air-tube concurrently serves as the spacingmeans. Such skeins with a generally circular or elliptical cross-sectionwith vertical air-tubes are less preferred to form a bank, but provide amore efficient use of available space in a reservoir than a rectangularskein.

Referring further to FIG. 2, the header 11 has front and rear wallsdefined by vertical (z-axis) edges 11′ and longitudinal (x-axis) edges13′; side walls defined by edges 11′ and transverse (y-axis) edges 13″;and a base 13 defined by edges 13′ and 13″.

The collection pan 20 is sized to snugly accommodate the base 13 above apermeate collection zone within the pan. This is conveniently done byforming a rectangular pan having a base 23 of substantially the samelength and width dimensions as the base 13. The periphery of the pan 20is provided with a peripheral step as shown in FIG. 2A, in which thewall 20′ of the pan terminates in a step section 22, having asubstantially horizontal shoulder 22″ and a vertical retaining wall 22′.

FIG. 2B is a bottom plan view of the lower face of header 13 showing theopen ends of the fibers 12′ prevented from touching each other bypotting resin. The geometrical distribution of fibers provides a regularperipheral boundary 14 (shown in dotted outline) which bounds theperipheries of the open ends of the outermost fibers.

Permeate flows from the open ends of the fibers onto the base 23 of thepan 20, and flows out of the collection zone through a permeatewithdrawal conduit 30 which may be placed in the bottom of the pan inopen flow communication with the inner portion of the pan. When the.skein is backwashed, backwashing fluid flows through the fibers and intothe substrate. If desired, the withdrawal conduit may be positioned inthe side of the pan as illustrated by conduit 30′. Whether operatingunder gravity alone, or with a pump to provide additional suction, itwill be apparent that a fluid-tight seal is necessary between theperiphery of the header 11 and the peripheral step 22 of the pan 20.Such a seal is obtained by using any conventional means such. as asuitable sealing gasket or sealing compound, typically a polyurethane orsilicone resin, between the lower periphery of the header 11 and thestep 22. As illustrated in FIG. 2, permeate drains downward, but itcould also be withdrawn from upper permeate port 45 u in the upperpermeate pan 43 u (see FIG. 9).

It will now be evident that a header with a circular periphery may beconstructed, if desired. Headers with geometries having still otherperipheries (for example, an ellipse) may be constructed in an analogousmanner, if desired, but rectangular headers are most preferred for easeof construction with multiple linear arrays.

Referring to FIGS. 9 and 2A, six rows of fibers 12 are shown on eitherside of a gas distribution line 52 which traverses the length of therows along the base of the fibers. The potted terminal end portions 12b″ open into permeate pan 43 b. Because portions 12 u′ and 12 b′ ofindividual fibers 12 are potted, and the fibers 12 are preferably from1% to 2% longer than the fixed distance between upper and lower headers41 u and 41 b, the fibers between opposed headers are generally parallelto one another, but are particularly parallel near each header. Alsoheld parallel are the terminal end portions 12 u″ and 12 b″ of thefibers which protrude from the headers with their open ends exposed. Thefibers protrude below the lower face of the bottom header 41 b, andabove the upper face of the upper header 41 u. The choice of fiberspacing in the header will determine packing density of the fibers nearthe headers, but fiber spacing is not a substantial considerationbecause spacing does not substantially affect specific flux duringoperation. It will be evident however, that the more fibers, the moretightly packed they will be, giving more surface area.

Since the length of fibers tends to change while in service, the extentof the change depending upon the particular composition of the fibers,and the spacing between the upper and lower headers is critical, it isdesirable to mount the headers so that one is adjustable in the verticaldirection relative to the other, as indicated by the arrow V. This isconveniently done by attaching the pan 43 u to a plate 19 havingvertically spaced apart through-passages 34 through which a threadedstud 35 is inserted and secured with a nut 36. Threaded stud 35 is in afixed mounting block 37.

The density of fibers in a header is preferably chosen to provide themaximum membrane surface area per unit volume of substrate withoutadversely affecting the circulation of substrate through the skein. Agas-distribution means 52 such as a perforated air-tube, provides airwithin the skein so that bubbles of gas (air) rise upwards whileclinging to the outer surfaces of the fibers, thus efficiently scrubbingthem. If desired, additional air-tubes 52′ may be placed on either sideof the skein near the lower header 41 b, as illustrated in phantomoutline, to provide additional air-scrubbing power. Whether the permeateis withdrawn from the upper header through port 45 u or the lower headerthrough port 45 b, or both, depends upon the particular application, butin all instances, the fibers have a substantially vertical orientation.

The vertical skein is deployed in a substrate to present a generallyvertical profile, but has no structural shape. Such shape as it doeshave changes continuously, the degree of change depending upon theflexibility of the fibers, their lengths, the overall dimensions of theskein, and the degree of movement imparted to the fibers by thesubstrate and also by the oxygen-containing gas from thegas-distribution means.

Referring to FIG. 10 there is illustrated a typical assembly referred toas a “Wall-mounted bank” which includes at least two side-by-sideskeins. indicated generally by reference numerals 40 and 40′ with theirfibers 42 and 42′; fibers 42 are potted in upper and lower headers 41 uand 41 b respectively; and fibers 42′ in headers 41 u′ and 41 b′;headers 41 u and 41 b are fitted with permeate collecting means 46 u and46 b respectively; headers 41 u′ and 41 b′ are fitted with permeatecollecting means 46 u′ and 46 b′ respectively; and, the skeins share acommon gas-distribution means 50. A “bank” of skeins is typically usedto retrofit a large, deep tank from which permeate is to be withdrawnusing a vacuum pump. In a large reservoir, several banks of skeins maybe used in side-by-side relationship within a tank. Each skein includesmultiple rows (only one row is shown) of fibers 42 and 42′ in upperheaders 41 u and 41 u′, and lower headers 41 b and 41 b′ respectively,and arms 51 and 51′ of gas-distribution means 50 are disposed betweenthe lower headers 41 b and 41 b′, near their bases. The upper headers 44u and 44 u′ are mounted by one of their ends to a vertical interiorsurface of the wall W of a tank, with mounting brackets 53 and 53′ andsuitable fastening means such as bolts 54. The wall W thus functions asa spacer means which fixes the distance between the upper and lowerheaders. Each upper header is provided with a permeate collection pan 43u and 43 u′, respectively, connected to permeate withdrawal conduits 45u and 45 u′ and manifolded to permeate manifold 46 u through whichpermeate being filtered into the collection pans is continuouslywithdrawn. Each header is sealingly bonded around its periphery, to theperiphery of each collection pan.

The skein fibers (only one array of which is shown for clarity) shown inthis perspective view have an elongated rectangular parallelpiped shapethe sides of which are irregularly shaped when immersed in a substrate,because of the random side-to-side displacement of fibers as they sway.An elongated rectangular parallelpiped shape is preferred since itpermits a dense packing of fibers, yet results in excellent scrubbing ofthe surfaces of the fibers with bubbles. With this shape, a skein may beformed with from 10 to 50 arrays of fibers across the longitudinal width‘w’ of the headers 41 u, 41 b, and 41 u′, 41 b′ with each array havingfibers extending along the transverse length ‘l’ of each header.Air-tubes on either side of a skein effectively cleanse the fibers ifthere are less than about 30 arrays between the air-tubes. A skeinhaving more than 30 arrays is preferably also centrally aerated asillustrated by the air-tube 52 in FIG. 9.

Thus, if there are about 100 fibers closely spaced-apart along thetransverse length ‘l’ of an array, and there are 25 arrays in a skein ina header of longitudinal width ‘w’, then the opposed terminal endportions of 2500 fibers are potted in headers 41 u and 41 b. The openends of all fibers in headers 41 b and 41 b′ point downwards intocollection zones in collection pans 43 b and 43 b′ respectively, andthose of all fibers in headers 41 u and 41 u′ point upwards intocollection zones in collection pans 43 u and 43 u′ respectively.Withdrawal conduits 45 u and 45 u′ are manifolded to permeate manifold46 u through which permeate collecting in the upper collection pans 43 uand 43 u′ is typically continuously withdrawn. If the permeate flow ishigh enough, it may also be withdrawn from the collection pans 43 b and43 b′ through withdrawal conduits 45 b and 45 b′ which are manifolded topermeate manifold 46 b. When permeate is withdrawn in the same plane asthe permeate withdrawal conduits 45 u, 45 u′ and manifold 46 u, and thetransmembrane pressure differential of the fibers is in the range from35-75 kPa (5-10 psi), manifold 46 u may be connected to the suction sideof a centrifugal pump which will provide adequate NPSH.

In general, the permeate is withdrawn from both the upper and lowerheaders, until the flux declines to so low a level as to require thatthe fibers be backwashed. The skeins may be backwashed by introducing abackwashing fluid through the upper permeate collection manifold 46 u,and removing the fluid through the lower manifold 46 b. Typically, from3 to 30 skeins may be coupled together for internal fluid communicationwith one and another through the headers, permeate withdrawal means andthe fibers; and, for external fluid communication with one anotherthrough an air manifold. Since the permeate withdrawal means is alsoused for backflushing it is generally referred to as a ‘liquidcirculation means’, and as a permeate withdrawal means only when it isused to withdraw permeate.

When deployed in a substrate containing suspended and dissolved organicand inorganic matter, most fibers of organic polymers remain buoyant ina vertical position. The fibers in the skein are floatingly buoyed inthe substrate with the ends of the fibers anchored in the headers. Thisis because (i) the permeate is essentially pure water which has aspecific gravity less than that of the substrate, and most polymers fromwhich the fibers are formed also have a specific gravity less than 1,and, (ii) the fibers are buoyed by bubbles which contact them. Fibersmade from ceramic, or, glass fibers are heavier than water.

Adjacent the skeins, an air-distribution manifold 50 is disposed belowthe base of the bundle of fibers, preferably below the horizontal planethrough the horizontal center-lines of the headers. The manifold 50 ispreferably split into two foraminous arms 51 and 51′ adjacent the basesof headers 41 b and 41 b′ respectively, so that when air is dischargedthrough holes in each portion 51 and 51′, columns of bubbles riseadjacent the ends of the fibers and thereafter flow along the fibersthrough the skeins. If desired, additional portions (not shown) may beused adjacent the bases of the lower headers but located on the outsideof each, so as to provide additional columns of air along the outersurfaces of the fibers.

The type of gas (air) manifold is not narrowly critical provided itdelivers bubbles in a preferred size range from about 1 mm to 25 mm,measured within a distance of from 1 cm to 50 cm from thethrough-passages generating them. If desired, each portion 51 and 51′may be embedded in the upper surface of each header, and the fiberspotted around them, making sure the air-passages in the portions 51 and51′ are not plugged with potting compound. If desired, additional armsof air-tubes may be disposed on each side of each lower header, so thatfibers from each header are scrubbed by columns of air rising fromeither transverse side.

The air may be provided continuously or intermittently, better resultsgenerally being obtained with continuous air flow. The amount of airprovided depends upon the type of substrate, the requirements of thetype of microorganisms, if any, and the susceptibility of the surfacesof the fibers to be plugged, there always being sufficient air toproduce desired growth of the microorganisms when operated in asubstrate where maintaining such growth is essential.

Referring to FIG. 11, there is schematically illustrated anotherembodiment of an assembly, referred to as a “stand-alone bank” ofskeins, two of which are referenced by numeral 60. The bank is referredto as being a “stand-alone” because the spacer means between headers issupplied with the skeins, usually because mounting the skeins againstthe wall of a reservoir is less effective than placing the bank inspaced-apart relationship from a wall. in other respects, the bank 60 isanalogous to the wall-mounted bank illustrated in FIG. 10.

Each bank 60 with fibers 62 (only a single row of the multiple,regularly spaced apart generally vertical arrays is shown for the sakeof clarity) is deployed between upper and lower headers 61 u and 61 b ina substrate ‘S’. The lower headers rest on the floor of the reservoir.The upper headers are secured to rigid vertical air tubes 71 and 71′through which air is introduced into a tubular air manifold identifiedgenerally by reference numeral 70. The manifold 70 includes (i) thevertical tubular arms 71 and 71′; (ii) a lower transverse arm 72 whichis perforated along the length of the lower header 61 b′ and securedthereto; the arm 72 communicates with longitudinal tubular arm 73, andoptionally another longitudinal arm 73′ (not shown) in mirror-imagerelationship with arm 73 on the far side of the headers; and (iii)transverse arms 74 and 74′ in open communication with 72 and 73; arms 74and 74′ are perforated along the visible transverse faces of the headers61 b an 61 b′, and 74 and 74′ may communicate with tubular arm 73′ if itis provided. The vertical air-tubes 71 and 71′ conveniently provide theadditional function of a spacer means between the first upper header andthe first lower header, and because the remaining headers in the bankare also similarly (not shown) interconnected by rigid conduits, theheaders are maintained in vertically and transversely spaced-apartrelationship. Since all arms of the air manifold are in opencommunication with the air supply, it is evident that uniformdistribution of air is facilitated.

As before, headers 61 u and 61 u′ are each secured in fluid-tightrelationship with collection zones in collection pans 63 u and 63 u′respectively, and each pan has withdrawal conduits 65 u and 65 u′ whichare manifolded to longitudinal liquid conduits 81 and 81′. Analogously,headers 61 b and 61 b′ are each secured in fluid-tight relationship withcollection zones in collection pans 63 b and 63 b′ respectively, andeach pan has withdrawal conduits 65 b and 65 b′ which are manifolded tolongitudinal conduits 82 and 82′. As illustrated, withdrawal conduitsare shown for both the upper and the lower headers, and both fore andaft the headers. In many instances, permeate is withdrawn from only anupper manifold which is provided on only one side of the upper headers.A lower manifold is provided for backwashing. Backwashing fluid istypically flowed through the upper manifold, through the fibers and intothe lower manifold. The additional manifolds on the aft ends of theupper and lower headers not only provides more uniform distribution ofbackwashing fluid but support for the interconnected headers. It will beevident that, absent the aft interconnecting upper conduit 81′, an upperheader such as 61 u will require to be spaced from its lower header bysome other interconnection to header 61 u′ or by a spacer strut betweenheaders 61 u and 61 b.

In the best mode illustrated, each upper header is provided with rigidPVC tubular nipples adapted to be coupled with fittings such as ells andtees to the upper conduits 81 and 81′ respectively. Analogously, eachlower header is connected to lower conduits 82 and 82′ (not shown)and/or spacer struts are provided to provide additional rigidity,depending upon the number of headers to be interconnected. Permeate iswithdrawn through an upper conduit, and all piping connections,including the air connection, are made above the liquid level in thereservoir.

The length of fibers (between headers) in a skein is generally chosen toobtain efficient use of an economical amount of air, so as to maintainoptimum flux over a long period of time. Other considerations includethe depth of the tank in which the bank is to be deployed, thepositioning of the liquid and air manifolds, and the convection patternswithin the tank, inter alia.

In another embodiment of the invention, a bioreactor is retrofitted withplural banks of skeins schematically illustrated in the elevational viewshown in FIG. 12, and the plan view shown in FIG. 13. The clarifier tankis a large circular tank 90 provided with a vertical, circular outerbaffle 91, a vertical circular inner baffle 92, and a bottom 93 whichslopes towards the center (apex) for drainage of accumulating sludge.Alternatively, the baffles may be individual, closely spaced rectangularplates arranged in outer and inner circles, but continuous cylindricalbaffles (shown) are preferred. Irrespective of which baffles are used,the baffles are located so that their bottom peripheries are located ata chosen vertical distance above the bottom. Feed is introduced throughfeed line 94 in the bottom of the tank 90 until the level of thesubstrate rises above the outer baffle 91.

A bank 60 of plural skeins 10, analogous to those in the bank depictedin FIG. 10, each of which skeins is illustrated in FIG. 9, is deployedagainst the periphery of the inner wall of the bioreactor with suitablemounting means in an outer annular permeate extraction zone 95′ (FIG.13) formed between the circular outer baffle 91 and the wall of the tank90, at a depth sufficient to submerge the fibers. A clarification zone91′ is defined between the outer circular baffle 91 and inner circularbaffle 92. The inner circular baffle 92 provides a vertical axialpassage 92′ through which substrate is fed into the tank 90. The skeinsform a dense curtain of fibers in radially extending, generally planarvertical arrays as illustrated in FIG. 9, potted between upper and lowerheaders 41 u and 41 b. Permeate is withdrawn through manifold 46u andair is introduced through air-manifold 80, extending along the innerwall of the tank, and branching out with air-distribution arms betweenadjacent headers, including outer distribution arms 84′ on either sideof each lower header 41 b at each end of the bank. The air manifold 80is positioned between skeins in the permeate extraction zone 95′ in sucha manner as to have bubbles contact essentially the entire surface ofeach fiber which is continuously awash with bubbles. Because the fibersare generally vertical, the air is in contact with the surfaces of thefibers longer than if they were arcuate, and the air is used mosteffectively to maintain a high flux for a longer period of time thanwould otherwise be maintained.

It will be evident that if the tank is at ground level, there will beinsufficient liquid head to induce a desirable liquid head under gravityalone. Without an adequate siphoning effect, a centrifugal pump may beused to produce the necessary suction. Such a pump should be capable ofrunning dry for a short period, and of maintaining a vacuum on thesuction side of from 50 cm (10″)-51 cm (20″) of Hg, or −35 kPa (−5 psi)to −70 kPa (−10 psi). Examples of such pumps rated at 18.9 L/min (5 gpm)@ 15″ Hg, are (i) flexible-impeller centrifugal pumps, e.g. Jabsco#30510-2003; (ii) air operated diaphragm pumps, e.g. Wilden M2; (iii)progressing cavity pumps, e.g. Ramoy 3561; and (iv) hosepumps, e.g.Waukesha SP 25.

The skein may also be potted in a header which is not a rectangularprism, preferably in cylindrical upper and lower headers in whichsubstantially concentric arrays of fibers are non-removably potted incylindrical permeate pans, and the headers are spaced apart by a centralgas tube which functions as both the spacer means and thegas-distribution means which is also potted in the headers. As before,the fibers are restrictedly swayable, but permeate is withdrawn fromboth upper and lower headers through a single permeate pan so that allconnections for the skein, when it is vertically submerged, are fromabove. Permeate is preferably withdrawn from the lower permeate panthrough a central permeate withdrawal tube which is centrally axiallyheld within the central gas (air) tube. The concentric arrays are formedby wrapping successive sheets of flat arrays around the′ centralair-tube, and gluing them together before they are potted. Thisconfiguration permits the use of more filtration surface area per unitvolume of a reservoir, compared to skeins with rectangular prismheaders, using the same diameter and length of fibers.

Referring to FIG. 14 there is schematically illustrated anotherembodiment of skein 180 in which rigid permeate tube 185 is heldconcentrically within a rigid air-supply tube 186 which is pottedaxially within skein fibers 112 held between opposed upper and lowerheaders 183 and 184 in upper and lower rings 120 u and 120 b which arein turn sealed in end-caps 181 and 182 respectively. For ease ofmanufacture, the lower end 185b of permeate tube 185 is snugly fittedand sealed in a bushing 187. The bushing 187 and end 185 b are theninserted in the lower end 86 b of the air supply tube 186 and sealed init so that the annular zone between the outer surface of permeate tube185 and the inner surface of air supply tube 186 will duct air to thebase of the fibers but not permit permeate to enter the annular zone.The air supply tube is then placed on an array and the array is rolledinto a spiral which is held at each end with rubber bands. The lower endof the roll is placed in a ring 120 b and a lower ring header is formedwith a finished header 184 as described above. It is preferred to use arelatively stiff elastomer having a hardness in the range from 50 ShoreA to about 20 Shore D, and most preferred to use a polyurethane having ahardness in the range from 50 Shore A to about 20 Shore D, measured asset forth in ASTM D-790, such as PTU-921 available from CanadianPoly-Tech Systems. To form the upper finished header 183 the air supplytube is snugly inserted through an O-ring held in a central bore in aplate such as used in FIG. 5, to avoid loss of potting resin from thering 120, and the fugitive resin and finishing resins poured and cured,first one then the other, in the ring. Lower finished header 184 isformed with intermediate portions 112 b′ embedded, and terminal portions112 b″ protruding from the header's aft face. Upper finished header 183is formed with intermediate portions 112 u′ embedded, and terminalportion 112 u″ protruding from the header's fore face. After thefinished headers 183 and 184 are formed and the fibers checked fordefects, the upper end 186 u of the air supply tube 186 is insertedthrough a central bore 188 in upper end-cap 181 and sealed within thebore with sealing compound or a collar 189. Preferably the permeate tube185, the air supply tube 186 and the collar 189 are all made of PVC sothat they are easily cemented together to make leak-proof connections.

As shown, permeate may be withdrawn through the permeate tube 185 fromthe permeate collection zone in the lower end-cap 182, and separatelyfrom the upper end-cap 181 through permeate withdrawal port 181 p whichmay be threaded for attaching a pipe fitting. Alternatively, thepermeate port 181 p may be plugged and permeate withdrawn from bothend-caps through the permeate tube 185.

Upper end 185 u of permeate tube 185 and upper end 186 u of air supplytube 186 are inserted through a T-fitting 201 through which air issupplied to the air supply tube 186. The lower end of 201 b of one ofthe arms of the T 201 is slip-fitted and sealed around the air supplytube. The upper end 201 u of the other arm is inserted in a reducingbushing 202 and sealed around the permeate tube.

Air supplied to intake 203 of the T 201 travels down the annular zonebetween the permeate tube and the air supply tube and exits throughopposed ports 204 in the lower portion of the air supply tube, justabove the upper face 184 u of the lower header 184. It is preferred tothread ports 204 to threadedly secure the ends of arms 141 to form asparger which distributed air substantially uniformly across and abovethe surface 184 u. Additional ports may be provided along the length ofthe vertical air supply tube, if desired.

EXAMPLE

Microfiltration of an activated sludge at 30° C. having a concentrationof 25 g/L (2.5% TSS) is carried out with a skein of polysulfone fibersin a pilot plant tank. The fibers are “air scrubbed” at a flow rate of12 CFM (0.34 m³/min) with a coarse bubble diffuser generating bubbles inthe range from about 5 mm to 25 mm in nominal diameter. The air issufficient not only for the oxidation requirements of the biomass butalso for adequate scrubbing. The fibers have an outside diameter of 1.7mm, a wall thickness of about 0.5 mm, and a surface porosity in therange from about 20% to 40% with pores about 0.2 μm in diameter, bothlatter physical properties being determined by a molecular weight cutoff at 200,000 Daltons. The skein which has 1440 fibers with a surfacearea of 12 m² is wall-mounted in the tank, the vertical spaced apartdistance of the headers being about 1% less than the length of a fiberin the skein. The opposed ends of the fibers are potted in upper andlower headers respectively, each about 41 cm long and 10 cm wide. Thefixing material of the headers is an epoxy having a hardness of about 70Shore D with additional upper an lower laminae of softer polyurethane(about 60 Shore A and 30 Shore D respectively) above and below the epoxylamina, and the fibers are potted to a depth sufficient to have theiropen ends protrude from the bottom of the header. The averagetransmembrane pressure differential is about 34.5 kPa (5 psi). Permeateis withdrawn through lines connected to the collection pan of eachheader with a pump generating about 34.5 kPa (5 psi) suction. Permeateis withdrawn at a specific flux of about 0.7 lm²h/kPa yielding about 4.8l/min of permeate which has an average turbidity of <0.8 NTU, which is aturbidity not discernible to the naked eye.

It will now be evident that the membrane device and basic separationprocesses of this invention may be used in the recovery and separationof a wide variety of commercially significant materials, some of which,illustratively referred to, include the recovery of water from groundwater containing micron and submicron particles of siliceous materials,preferably “gas scrubbing” with carbon dioxide; or, the recovery ofsolvent from paint-contaminated solvent. In each application, the choiceof membrane will depend upon the physical characteristics of thematerials and the separation desired. The choice of gas will depend onwhether oxygen is needed in the substrate.

In each case, the simple process comprises, disposing a skein of amultiplicity of hollow fiber membranes, or fibers each having alength >0.5 meter, together having a surface area >1 m², in a body ofsubstrate which is unconfined in a modular shell, so that the fibers areessentially restrictedly swayable in the substrate. The substrate istypically not under pressure greater than atmospheric. The fibers have alow transmembrane pressure differential in the range from about 35 kPa(0.5 psi) to about 350 kPa (50 psi), and the headers, the terminalportions of the fibers, and the ends of the fibers are disposed inspaced-apart relationship as described hereinabove, so that by applyinga suction on the aft face of at least one of the headers, preferablyboth, permeate is withdrawn through the collection means in which eachheader is mounted in fluid-tight communication.

Having thus provided a general discussion, and specific illustrations ofthe best mode of constructing and deploying a membrane device comprisinga skein of long fibers in a substrate from which a particular componentis to be produced as permeate, how the device is used in a gas-scrubbedskein, and having provided specific illustrative systems and processesin which the skein is used, it is to be understood that no unduerestrictions are to be imposed by reason of the specific embodimentsillustrated and discussed, and particularly that the invention is notrestricted to a slavish adherence to the details set forth herein.

We claim:
 1. An apparatus for treating a multicomponent liquid substratewhile leaving particulate matter therein, comprising, (a) anon-pressurized reservoir for containing the substrate; (b) a skein ofhollow fiber filtering membranes immersed in the substrate wherein themembranes are disposed generally vertically between upper and lowerheaders such that (i) outsides of ends of the membranes are sealinglysecured in closely spaced-apart relationship in the headers, (ii) lumensof the membranes are in fluid communication with at least one permeatecollection means, and, (iii) the membranes have a length between opposedsurfaces of the headers such that the membranes may move against eachother but wherein the length is less than 5% greater than the distancebetween opposed surfaces of the headers; (c) a pump in fluidcommunication with the lumens of the membranes, the pump operable toapply suction to the lumens of the membranes to draw a component of thesubstrate as permeate through the membranes; and, (d) an aeration systemhaving a gas distributor for discharging air directly into the substratewithin the skein.
 2. The apparatus of claim 1 wherein the length is atleast 0.1% but less than 1% longer than the distance between the opposedsurfaces of the headers.
 3. The apparatus of claim 1 wherein the gasdistributor comprises a pipe with holes for discharging air throughoutits circumference.
 4. The apparatus of claim 3 wherein the upper andlower headers are cylindrical and the gas distributor is a centrallylocated gas tube which also spaces the lower and upper headers relativeto one another.
 5. An apparatus for treating a multicomponent liquidsubstrate while leaving particulate matter therein, comprising, (a) anon-pressurized reservoir for containing the substrate; (b) a skein ofhollow fiber filtering membranes immersed in the substrate wherein themembranes are disposed generally vertically between upper and lowerheaders such that (i) outsides of ends of the membranes are sealinglysecured in closely spaced-apart relationship in the headers, (ii) lumensof the membranes are in fluid communication with at least one permeatecollection means, and, (iii) the membranes have a length between opposedsurfaces of the headers such that the membranes may move against eachother but wherein the length is less than 5% greater than the distancebetween opposed surfaces of the headers; (c) a pump in fluidcommunication with the lumens of the membranes, the pump operable toapply suction to the lumens of the membranes to draw a component of thesubstrate as permeate through the membranes; and, (d) a gas distributionsystem having through-passages adapted to discharge bubbles to risethrough and around the skein, the gas distribution system including oneor more gas tubes which space the first and second headers apart andalso duct air to the through-passages.
 6. The apparatus of claim 5wherein the upper and lower headers are cylindrical and the one or moregas tubes are a single centrally located gas tube.
 7. An apparatus fortreating a multicomponent liquid substrate while leaving particulatematter therein, comprising, (a) a non-pressurized reservoir forcontaining the substrate; (b) a plurality of hollow fiber filteringmembranes immersed in the substrate wherein the membranes are disposedgenerally vertically between upper and lower headers such that (i)outsides of ends of the membranes are sealingly secured to the headersin a closely spaced apart relationship, (ii) lumens of the membranes arein fluid communication with at least one permeate collection means, and,(iii) said membranes having a length between opposed surfaces of theheaders such that the membranes may move against each other but whereinthe length is less 5% greater than the distance between opposed surfacesof the headers; (c) a pump in fluid communication with said lumens ofsaid membranes, said pump operable to apply a suction to the lumens ofthe membranes to draw a component of the substrate as permeate throughsaid membranes; and, (d) a gas distribution system havingthrough-passages through the lower header to discharge bubbles into thesubstrate above the lower header.
 8. The apparatus of claim 7 whereinthe gas distribution system comprises a manifold, and thethrough-passages are connected to a source of pressurized gas throughthe manifold.
 9. The apparatus of claim 7 wherein the through-passagesconnect to a plenum directly below the lower header into which air maybe introduced.
 10. The apparatus of claim 9 wherein openings of thethrough-passages are located between membranes.
 11. The apparatus ofclaim 10 wherein the length is at least 0.1% but less than 1% longerthan the distance between the opposed surfaces of the headers.
 12. Theapparatus of claim 7 wherein the length is at least 0.1% but less than1% longer than the distance between the opposed surfaces of the headers.